Trans effects at the sn-glycerol-3-phosphate dehydrogenase locus (Gpdh) of Drosophila melanogaster give rise to an increase in GPDH activity and mRNA from the wild-type allele in heterozygotes with some low-activity alleles. Either the low-activity alleles that induce the effect have a defective P-element inserted between the promoter and a downstream intronic enhancer element or the promoter region is deleted. The trans effect is pairing dependent, characteristic of transvection at some other loci. The defective P-elements that mediate transvection are located between the promoter and at least up to 6 bp downstream of the transcription start site. Transvection at Gpdh appears similar to the "enhancer action in trans" mode at the yellow locus.